You’ve probably read it online somewhere or in marketing communications by the major three lighting manufacturers: the stopping ability of a strobe is determined by its t.1 flash duration. t.1 is defined as the strobes flash duration above 10% output, whereas t.5 means above 50%. But what does that really mean?
I am not sure if it was Broncolor who came up with the idea to sell photographers on the flash duration, but they sure did quite a good job with convincing the majority of photographers that it matters, and why they need to invest into Bron lights, if they want to stop motion. It was in fact so successful, as a message, that Profoto jumped on that marketing train as well a few years ago, claiming that their lights are faster and better for stopping motion – this was the megapixel race allover again. So why is this argument about flash duration complete BS? Let’s look at the factors that influence exposure, to understand why this marketing speak is completely irrelevant to the type of photography you do.
So what influences your exposure? Simple: ambient light, artificial lights (LED, HMI, strobe etc.), shutter speed, aperture of the lens, ISO. Let’s break these down in terms of how they influence the stopping ability.
Ambient Light can be the sun, or any other lightsource. If you have enough ambient light, you can stop motion by increasing the shutter speed. Keep in mind that whenever there is ambient light contributing to your exposure, the only way for you to freeze motion is through shutter speed!
Artificial lights (such as strobes or LED lights) are equally dependent on the shutter speed to stop motion. The only exception is when there is no ambient light contributing to the exposure, the actual flash duration becomes effectively your shutter speed (think of shooting in a dark room, or with closed down aperture where an exposure without the strobe results in a black image)
Shutter speed is the only constant influence on all of your images, regardless of where or how you take them, which always contributes to your ability to stop motion. Faster shutter, better stopping. It is really that simple.
Aperture has no influence on your ability to stop motion, other than a wider aperture allowing you to shoot at faster shutter speeds with less light needed (or when you want to capture ambient light, for example). If you take an image at 1/30th of a second at f/2.8, and then at f/11 the stopping ability of the camera remains the same, only the overall exposure will change, with one image being darker than the other by 4 stops.
ISO essentially is like an electronic aperture without influencing the depth of field (but at the expense of added noise the higher you go on your ISO). ISO is essentially the amplification of the signal that comes from your sensor, amplifying all the data you want, but also all the data you don’t (aka noise). This again has no influence on the stopping ability of your camera unless it is paired with faster shutter speed (higher iso=faster shutter and/or smaller aperture on your lens)
Think of it this way: before we had High Speed Flash Sync, cameras were stuck at 1/125th or 1/160th maximum shutter speed when syncing to strobes. If you are a studio photographer and have lots of flash power, you could close down your aperture, thus eliminating any ambient light hitting the sensor, and then use the ability of the strobe to stop motion, if the strobe was firing faster than 1/120th at t.1. Because no ambient light hits your sensor during exposure, that movement that you would otherwise not stop with ambient light present, is not “seen” by the sensor, as the light will only be bright enough during the time the strobe fires. So if your strobe’s t.1 is around 1/500th of a second at a higher power setting, then you would essentially shoot with a 1/500th shutter. That is the only time when this applies.
Now, I am sure you start to see why flash duration doesn’t matter, if you want to shoot with open apertures outside of a completely dark room, balance ambient and flash light, or if you have access to technology that syncs to your camera’s shutter at any shutter speed. This whole flash duration talk is really all about the limitation of past times, where high speed flash sync was not possible, but even then, if you wanted to use this efficiently, you are quite limited in what and where you can shoot (remember, ambient light is your enemy if you want to stop motion with flash duration alone).
So why are these guys still talking about flash duration, if it really doesn’t matter? Well, marketing talk is one thing. They have been preaching to photographers for years that flash duration is what matters (although, as you can read above, this really didn’t make a difference for almost all applications), and now simply can’t backtrack and say “hey, we took you for a ride and those 10s of thousands you spent on our gear was just our way of milking you“.
The problem with all these strobes out there today, and I am talking Elinchrom Ranger, Profoto B1/B2, and recently Broncolor Siros, as well as some Chinese brands, is that they have been optimized for short flash duration, making it impossible for them to offer high speed flash sync at any shutter speed AND any power setting, or with more than 800Ws. Priolite is the only strobe on the market with up to 1000Ws per head and the ability to sync to Canon, Nikon, Pentax, and Sony at up to 1/8000th at any power setting.
Keep in mind that most strobes get slower t.1 times the more power you use, so the whole argument of short flash duration does not apply to closed down apertures (if you drop ambient, you drop flash as well, so each stop you drop you need to add 1 extra stop of output to your lights – see why photographers used to shoot with large 2400Ws or more packs?). Unless you shoot at low output, these strobes aren’t that fast, and you will quickly find yourself in the position where your 1/2000th shutter on your camera is faster than the flash duration you can achieve with these strobes, with only one difference: these strobes may not allow you to sync to your light at 1/8000th with the power output you need to take your image. In other words, stopping motion through flash duration is a compromise that made sense when it was the only solution before High Speed Flash sync, but there is no reason today to compromise, when a vastly superior approach exists, and which also enables you to take pictures you simply can’t create with other tools.
Flash duration doesn’t matter, the ability of the strobe to sync to your camera at any shutter speed and power setting is the only thing that does!